1. Field of the Invention
The invention relates to a motor controller, an electric vehicle that is provided with a motor controller, and a method for estimating a heat stress to which a switching element converting output electric power of an electric power supply into motor-driving electric power is subjected.
2. Description of Related Art
A motor controller is provided with a switching element that converts output electric power of an electric power supply into motor-driving electric power. Hereinafter, the switching element that converts the source electric power into the motor-driving electric power will be simply referred to as a “switching element”. The switching element is a main component of an inverter that converts direct-current electric power of the electric power supply into alternating-current electric power or a main component of a boost converter that boosts an electric power supply voltage. Some motors have a driving electric power of at least 10 kilowatts, and a large current flows through the switching element for the motor that has such a large output. Accordingly, the switching element is prone to having a high heating value and having a high temperature. In the meantime, an electric vehicle frequently repeats start and stop and frequently repeats acceleration and deceleration, and thus the temperature of the switching element frequently changes. Not only the electric vehicle but also a motor for driving a joint of a robot frequently repeats start and stop and frequently repeats acceleration and deceleration, and thus the temperature of the switching element frequently changes. Due to the change in the temperature of the switching element, the switching element (or components around the switching element) repeats expansion and contraction. The switching element (or the components around the switching element) is subjected to damage during each expansion or contraction. In this specification, the damage to the switching element that is attributable to the temperature change will be referred to as a “heat stress”. A performance of the switching element is reduced when the heat stress that the switching element is subjected to exceeds a predetermined amount.
Japanese Patent Application Publication No. 2012-19587 (JP 2012-19587 A) discloses a technique for suppressing an excessive heat stress. The motor controller that is disclosed in JP 2012-19587 A is a device mounted in an electric car and supplying electric power to a traveling motor. The technique that is disclosed in JP 2012-19587 A is as follows. The motor controller that is disclosed in JP 2012-19587 A counts a set of a rise and a fall in the temperature of the switching element as one heat stress and obtains a difference between a minimum value and a maximum value of the temperature of the switching element therebetween (the amount of temperature change). In addition, the motor controller obtains a distribution (actual result distribution) in which the amount of the temperature change is on a horizontal axis and a frequency of appearance of each temperature conversion amount (the number of occurrences) is on a vertical axis. The motor controller has a desirable reference distribution of the amount of the temperature change and the frequency of appearance (the number of occurrences) stored in advance, and intensifies cooling of the switching element such that the actual result distribution becomes close to the reference distribution when the actual result distribution is shifted toward a side of a larger amount of the temperature change with respect to the reference distribution.
In the technique according to JP 2012-19587 A, the set of the rise in the switching element temperature and the fall in the switching element temperature is counted as one heat stress. Multiple maximum points and multiple minimum points appear in a time-dependent change in the temperature of the switching element. Accordingly, the temperatures at the minimum points which are adjacent to each other on a time axis are not necessarily equal to each other. In the case of a turn toward another temperature rise from a temperature that is higher than a temperature at an initiation of a temperature rise, the temperature of the switching element might fall to a temperature that is lower than the temperature at the initiation of the temperature rise. Then, the damage (that is, the heat stress) to which the switching element is subjected might be different in a temperature-rising phase and a temperature-lowering phase. The technique that is disclosed in JP 2012-19587 A does not take this difference into account.